The present invention relates to milling tools for rotary machining of objects; more particularly, to hollow milling tools for forming generally cylindrical features on the outer surfaces of objects; and most particularly, to a hollow milling tool having blades which are adjustable to provide a range of cylinder cutting diameters, wherein the axial and radial rake angles of the cutting edges with respect to the centerline of the workpiece, and the radial and axial clearance angles of the cutting edges with respect to the cut surfaces of the workpiece, are constant over the range of cutting diameters of the milling tool.
Hollow milling tools, also referred to herein as hollow mills, are well known in the art of machining, See, for example, U.S. Pat. No. 887,992 issued May 19, 1908 to Wonderly; U.S. Pat. No. 1,244,665 issued Oct. 30, 1917 to Watson; and U.S. Pat. No. 1,445,448 issued Feb. 13, 1923 to Reisinger. In general form, a hollow mill includes a longitudinal, generally cylindrical body having a shank at one end for insertion into a driven chuck and having an axial bore terminating in an opening at the other end into which a workpiece may be entered progressively during machining thereof. Typically, the outer surface of the body is provided with at least one slot cut at specific radial and axial angles with respect to the centerline of the tool for receiving and mounting a cutting member generally axially and diagonally to form a work zone adjacent to the axial opening. The cutting member may be a blade formed with one or more desired angles of the cutting edges, or the member may be a holder for a blade insert such as a formed carbide cutter. Preferably, a mill includes a plurality of identical slots and associated cutting members disposed in radial symmetry about the axis. The bottom of each slot is inclined to the tool axis, typically at an included angle of 20xc2x0, so that virtual extensions of the cutting members intersect at the axis. Each cutting member is held in its respective slot by a positive clamping mechanism. Typically, the mill body includes threads on its outer surface and is provided with a threaded knurled ring which bears on the non-cutting ends of the cutting members, as they extend beyond the body, to adjust the longitudinal position of the cutting members to effect a desired diameter of the cutting edges.
Within the known art, a cutting tool is said to have xe2x80x9crakexe2x80x9d when the tool face, or surface against which the chips bear as they are severed, is inclined from the axis of the tool for either increasing or diminishing the keenness or bluntness of the edge. If the inclination of the tool face is such as to make the cutting edge keener than when the rake angle is zero, the rake is said to be positive. Conversely, if the effect is to make the edge less keen than when the rake is zero, the rake is said to be negative. The magnitude of the rake is measured by two angles commonly known in the art as the xe2x80x9caxial rake anglexe2x80x9d and the xe2x80x9cradial rake angle.xe2x80x9d
In a hollow mill cutting tool, the angle formed between a plane including the forward surface (tool face) of the cutting member and the centerline of the workpiece (coincident with the axis of the mill) is the axial rake angle. The angle formed between the position of the tip (intersection of the axial and radial cutting edges) of the cutting member as it lies on the workpiece and the centerline of the workpiece (again coincident with the axis of the mill) is the radial rake angle.
If the hollow mill has zero cutting rake, the tool face of the cutting member is parallel to the centerline of the workpiece and the tip of the cutting member lies on the centerline of the workpiece. If the mill has positive axial rake, the tool face is inclined toward the direction of rotation of the tool. If the mill has positive radial rake, the tool tip lies on the workpiece ahead of the centerline of the workpiece.
In the known art, axial and radial rake angles are established by cutting the slots in the mill body at the desired angles such that a cutting member when mounted will assume the correct rake angles. Further, the cutting members are provided at a correct length such that each cutting tip is disposed at a desired location with respect to the centerline of the workpiece, typically on or slightly ahead of a plane through the centerline of the workpiece. xe2x80x9cAheadxe2x80x9d corresponds to xe2x80x9cin the direction of rotation of the tool.xe2x80x9d
Other angles of importance are the clearance angles behind the radial and axial cutting edges. Typically, the included angle in the cutting edges is slightly less than 90xc2x0 to promote clean cutting and to prevent possible binding of the tangent side of the cutter with the workpiece. This is especially important in positive radial rake mills wherein the radial clearance angle must at least equal the positive radial rake angle; otherwise, the cutting edge cannot make contact with the workpiece.
A problem arises in known mills having adjustable cutters mounted with positive or zero axial rake and positive radial rake, however. Typically, the slots in the mill body are cut such that the forward surface of the slot lies in a plane either at zero axial rake or at a positive axial rake. A blade or insert holder installed in such a slot assumes the fixed axial rake angle of the slot at all cutting diameters of the blade, but the radial rake angle and the radial clearance angle must change as the blade is moved diagonally along the slot to change the cutting diameter of the tool. Therefore, a prior art positive rake angle tool has almost no diameter adjustability, the range being only about 0.050 inches or less. Each prior art positive rake hollow mill must be provided with a dedicated set of cutting members specifically shaped to engage a workpiece at a particular very narrow range of diameters. Cutting members are not exchangeable between prior art mills, and a large number of different-sized mills and/or cutters are required to mill a wide range of diameters.
What is needed is a hollow mill which is readily adjustable for cutting over a wider range of diameters at a constant axial rake angle, constant radial rake angle, and constant radial and axial clearance angles.
It is a principal object of the invention to provide an improved adjustable hollow mill wherein the axial rake angle, radial rake angle, and axial and radial clearance angles are constant over the range of adjustment of the mill.
It is a further object of the invention to provide an improved adjustable hollow mill having constant axial rake angle, constant radial rake angle, and constant clearance angles wherein the ratio of the minimum diameter to the maximum diameter which may be milled is at least 1:2.
It is a still further object of the invention to provide an improved adjustable hollow mill having constant axial rake angle, constant radial rake angle, and constant axial and radial clearance angles wherein the range of diameter size adjustability is at least 0.25 inches.
Briefly described, an adjustable hollow milling tool embodying the invention includes a longitudinal body having a shank at one end for insertion into a rotationally-driven chuck and having an axial bore terminating in an opening at the other end into which a workpiece, either rotating or stationary, may be entered progressively during machining thereof. The body is provided with at least one slot, and preferably three or more slots, cut at a negative axial angle with respect to the centerline of the tool for receiving and mounting a cutting member. The slot intersects the front face of the tool at a location ahead of a plane including the centerline of the tool. The cutting member may be a blade formed with one or more desired angles of the cutting edge, and preferably the member is a holder for a cutter insert such as a formed carbide indexable insert. Preferably, the insert is mounted in the insert holder such that the forward face of the insert forms an included angle with the forward surface of the insert holder which is at least equal and opposite to the negative axial rake angle of the slot, so that the forward face of the insert is at least parallel to or ahead of a plane containing the centerline (zero or positive axial rake of the cutter insert). Each insert holder is held in its respective slot by a positive clamping mechanism. The mill body is threaded on its outer surface and is provided with a threaded knurled ring which bears on the non-cutting ends of the insert holders, as they extend beyond the body, to move the insert holders in the slots to a desired cutting diameter of the cutting edges. The negative axial rake angle of the slot and the point of intersection of the slot with the tool face are selected such that the cutting edge is positioned at a desired positive radial rake angle and neutral or positive axial rake angle with positive radial and axial clearance angles, such that the radial rake angle, axial rake angle, and radial and axial clearance angles are constant over all diameters through which the milling tool is adjustable.